Protective cellular immune responses against foreign antigens rely on functional T lymphocytes. The development, activation and differentiation of T cells are controlled by transcription factors in different combinations. GA binding protein (GABP) is a ubiquitously expressed Ets family transcription factor, and functional GABP consists of two different subunits, GABP1 and GABP2, which mediate DNA binding and transactivation, respectively. Previously we have demonstrated that GABP critically regulates the expression of interleukin-7 receptor 1 (IL-7R1) chain in T cells, suggesting a key role of GABP in the immune system. Whereas complete inactivation of the GABP1 gene caused pre- implantation lethality, we have generated a GABP1 mutant mouse strain that is hypomorphic and has prolonged survival during embryogenesis. In our preliminary studies using fetal liver cells from the GABP1 mutant embryos to reconstitute lymphoid compartment in Rag2-deficient mice, we found that GABP has essential roles in thymocyte development and T cell-mediated immune responses. This research project aims to construct a hierarchical and interactive gene regulatory network under the control of GABP in T cells. Our long-term goal is to elucidate how GABP programs T lymphopoiesis and T cell immune responses through regulating its target genes and interacting with other key factors. This information will provide a scientific basis for modulating outcomes of hematopoietic stem cell differentiation and immune responses under physiological and pathological conditions. Specific Aim. To identify direct GABP target genes in T lymphocytes and to establish a GABP- controlled gene regulatory network programming T cell biology. Taking advantage of our GeneChip microarray analysis data, we will extend our studies to identify GABP direct target genes using approaches of comparative genomics and chromatin immunoprecipitation assay (ChIP) coupled with a mouse promoter array (ChIP-on-chip). We will then map the precise locations of GABP-binding elements and investigate their functional importance in activating/repressing respective genes. With this knowledge, a gene regulatory network in T cells can be constructed, and this will provide a platform for a broader investigation of the interactive transcriptional control of thymocyte development and T cell functions. Reconstitution of the immune system in individuals with inherited and acquired immunodeficiency remains a major challenge, and a better understanding of the molecular mechanism by which hematopoietic stem cells differentiate to lymphoid lineages is fundamental to improve strategies for immune system reconstitution. This project will generate insights into how GABP, a transcription factor, contributes to specifying hematopoietic stem cells to the T-lineage cells, and how GABP modulates the cellular immune response. This knowledge will provide a scientific basis for manipulating outcomes of hematopoietic stem cell differentiation and immune response under physiological and pathological conditions.